The invention relates to a process for regenerating periodic acid and to a process of oxidizing carbohydrates with periodic acid with regeneration and recovery of the periodic acid.
Periodic acid is extensively used for oxidizing vicinal diols resulting in dialdehydes according to the following reaction:
R1xe2x80x94CHOHxe2x80x94CHOHxe2x80x94R2+IO431xe2x86x92R1xe2x80x94CHxe2x95x90O+Oxe2x95x90CHxe2x80x94R2+IO3xe2x88x92+H2O
Instead of periodate (IO4xe2x88x92), the actual oxidizing species is para- or meta-periodate (HnIO6(5-n)xe2x88x92), which results from reaction of periodic acid with water:
HIO4+2H2Oxe2x86x92H5IO6xe2x86x92H++H4IO6xe2x88x92
In the present description, all such oxoiodate species of heptavalent iodine, whether in neutral or in deprotonated form, are referred to as periodate. Usually, the two groups R1 and R2 are part of the same ring system, such as in carbohydrates. The most common process in which periodate is used as an oxidizing agent is the oxidation of starch to dialdehyde starch (DAS), which is used as a wet strength additive for paper, or can be further oxidized to calcium-binding dicarboxy starch.
As periodate is an expensive oxidizing agent, the spent oxidizing agent, i.e. iodate, should be regenerated to periodate. This is usually done by oxidation with hypochlorite, as described by McGuire and Mehltretter, Die Stxc3xa4rke, 23, (1971) 42-45. According to this method, spent iodate is treated with 1.5 mol equivalents (3.0 redox equivalents) of sodium hypochlorite at pH 13-14 and at 90-95xc2x0 C. for 40 min, resulting in an average recovery of 97.6% periodate, EP-A-913358 (DSM) discloses the preparation of sodium para-periodate from iodine, iodide or iodate using alkaline sodium hypochlorite in the presence of other ions such as chloride or sulfate WO 98/27118 (Avebe) describes oxidation of starch with periodate, and recovery of periodate by oxidation of the iodate with ozone.
The prior art methods for periodate recovery have drawbacks in that they are performed at high pH, so that the product cannot be directly used in a process performed under acidic conditions. Moreover, processes using hypohalite are undesired, since they produce substantial amounts of chlorinated by-products.
It was found according to the invention that the regeneration of periodate can be improved using persulfuric acid as the oxidizing species. The persulfuric acid or peroxosulfate may have any form, and is preferably a mixed salt such as commercially available Oxone(copyright) (2KHSO5.KHSO4.K2SO4). The regeneration proceeds not only for producing periodate from iodate, preferably an iodate-containing solution, but also from iodide or other iodine species having lower oxidation states than the oxidation slate in periodate (+7).
The process of the invention allows short regeneration times and low temperatures (0-20xc2x0 C.), although higher temperatures, up to about 60xc2x0 C. can be used as well. The process also avoids the formation of halogenated products of the periodate oxidation and the production of halide byproducts. Moreover, the oxidation can be carried out at acidic pH, as low as pH 3, up to about pH 8. This is especially advantageous where the periodate is used in an oxidation process at low pH, such as the oxidation of polysaccharides to produce dialdehyde polysaccharides, as this avoids the need for a pH adjustment between oxidation and regeneration. This allows both steps to be performed simultaneously and/or using the same reactor, if the reaction product of the oxidation is compatible with the peroxosulfate.
Thus, according to an embodiment of the present invention, the process can be carried out in the same reactor as the process in which the periodate is used for oxidizing a substrate, in particular a xcex1,xcex2-diol. In another embodiment of the invention, the regeneration reaction is carried out simultaneously with the oxidation reaction but physically separated from it. For example, a two-compartment system can be designed, comprising a reaction compartment and a regeneration compartment, with a filter being provided between the two compartments. The filter can e.g. be a polymeric membranes and can serve to keep the (insoluble) reaction product of the oxidation reaction, such as dialdehyde starch, in the reaction compartment and the per-oxosulfate in the regeneration compartment. In these embodiments, the reaction can be performed continuously or semi-continuously, and the periodate can be used in sub-stoichiometric (catalytic) amounts, e.g. 0.05-0.5 equivalents. These embodiments require a careful control of peroxosulfate addition so as to avoid an excess of peroxosulfate to be present with the substrate.
Alternatively, the process can be performed batch-wise. A stoichiometrical or near-stoichiometrical amount of periodate can then be used. The iodate produced in the oxidation reaction can be separated from the reaction mixture by precipitation, if necessary by increasing the pH, and the iodate can then be regenerated in a separate reactor following the process of the invention. If the substrate and the reaction product are insoluble, such as with cellulose fibers, the can be simply separated from the action mixture by filtration or decanting, before optional precipitation of the spent iodate.
The regenerated periodate is particularly suitable for the oxidation of carbohydrates to produce carbohydrate dialdehydes, such as members of the starch family (starch, amylose, amylopectin, hydrolysates and derivatives thereof), cellulose, other glucans, galactomannans (guar, locust bean gum), fructans (inulin), xylans, and the like, and alkylated, carboxyalkylated, hydroxyalkylated and other derivatives thereof, provided they contain vicinal diol groups (xe2x80x94CHOHxe2x80x94CHOHxe2x80x94). Starch and starch derivatives and cellulose are especially preferred. It is observed that where the carbohydrate oxidation product is soluble in the reaction medium such as with low molecular weight carbohydrate dialdehydes, an effective separation of oxidation product (dialdehyde) and iodate can be achieved by precipitating the iodate, e.g. by addition of potassium or magnesium ions or an organic solvent such as ethanol. The precipitated iodate is then reoxidized by redissolution and oxidation as described above. When the oxidation and regeneration are carried out simultaneously, the pH is preferably between 3.5 and 6, most preferably between 4 and 5. The temperature can be kept low, e.g. between 0 and 10xc2x0 C.
The dialdehyde oxidation products thus obtained can be used for various purposes, e.g., as a crosslinking agent, as an additive e.g. in glues, coatings, thickeners, and the like, or as a carrier material e.g. for proteins. The dialdehydes are also suitable as wet strength agents, optionally after partial further oxidation to products having both aldehyde and carboxyl groups (see WO 00/26257). They may also be used as starting material for producing dicarboxy carbohydrates, which are suitable e.g. as calcium binding agents and as a raw material for producing superabsorbent materials. The dicarboxy carbohydrates can be conveniently prepared from the dialdehyde carbohydrates by oxidation with usual oxidizing agents, in particular sodium chlorite.
The dicarboxy starches that can be obtained from the dialdehyde starches prepared by using extended periodate oxidation periods (e.g. 4-8 days) have an unexpectedly high calcium-binding power, as measured by their sequestering capacity (SC) of at least 2.8, especially at least 3.0 mmol Ca/g.